586 research outputs found
A Report on the X-ray Properties of the tau Sco Like Stars
An increasing number of OB stars have been shown to possess magnetic fields.
Although the sample remains small, it is surprising that the magnetic and X-ray
properties of these stars appear to be far less correlated than expected. This
contradicts model predictions, which generally indicate that the X-rays from
magnetic stars to be harder and more luminous than their non-magnetic
counterparts. Instead, the X-ray properties of magnetic OB stars are quite
diverse.
Sco is one example where the expectations are better met. This bright
main sequence, early B star has been studied extensively in a variety of
wavebands. It has a surface magnetic field of around 500 G, and Zeeman Doppler
tomography has revealed an unusual field configuration. Furthermore, tau Sco
displays an unusually hard X-ray spectrum, much harder than similar,
non-magnetic OB stars. In addition, the profiles of its UV P Cygni wind lines
have long been known to possess a peculiar morphology.
Recently, two stars, HD 66665 and HD 63425, whose spectral types and UV wind
line profiles are similar to those of Sco, have also been determined to
be magnetic. In the hope of establishing a magnetic field - X-ray connection
for at least a sub-set of the magnetic stars, we obtained XMM-Newton EPIC
spectra of these two objects. Our results for HD 66665 are somewhat
inconclusive. No especially strong hard component is detected; however, the
number of source counts is insufficient to rule out hard emission. longer
exposure is needed to assess the nature of the X-rays from this star. On the
other hand, we do find that HD 63425 has a substantial hard X-ray component,
thereby bolstering its close similarity to tau Sco.Comment: MNRAS, accepte
Profile Shapes for Optically Thick X-ray Emission Lines from Stellar Winds
We consider the consequences of appreciable line optical depth for the
profile shape of X-ray emission lines formed in stellar winds. The hot gas is
thought to arise in distributed wind shocks, and the line formation is
predominantly via collisional excitation followed by radiative decay. Such
lines are often modelled as optically thin, but the theory has difficulty
matching resolved X-ray line profiles. We suggest that for strong lines of
abundant metals, newly created photons may undergo resonance scattering,
modifying the emergent profile. Using Sobolev theory in a spherically symmetric
wind, we show that thick-line resonance scattering leads to emission profiles
that still have blueshifted centroids like the thin lines, but which are
considerably less asymmetric in appearance. We focus on winds in the
constant-expansion domain, and derive an analytic form for the profile shape in
the limit of large line and photoabsorptive optical depths. Our theory is
applied to published {\it Chandra} observations of the O star Pup.Comment: ApJ, in pres
A propelling neutron star in the enigmatic Be-star ~Cassiopeia
The enigmatic X-ray emission from the bright optical star,
Cassiopeia, is a long-standing problem. Cas is known to be a binary
system consisting of a Be-type star and a low-mass ()
companion of unknown nature orbiting in the Be-disk plane. Here we apply the
quasi-spherical accretion theory onto a compact magnetized star and show that
if the low-mass companion of Cas is a fast spinning neutron star, the
key observational signatures of Cas are remarkably well reproduced.
Direct accretion onto this fast rotating neutron star is impeded by the
propeller mechanism. In this case, around the neutron star magnetosphere a hot
shell is formed that emits thermal X-rays in qualitative and quantitative
agreement with observed properties of the X-ray emission from Cas. We
suggest that Cas and its analogs constitute a new subclass of Be-type
X-ray binaries hosting rapidly rotating neutron stars formed in supernova
explosions with small kicks. The subsequent evolutionary stage of Cas
and its analogs should be the X Per-type binaries comprising low-luminosity
slowly rotating X-ray pulsars. The model explains the enigmatic X-ray emission
from Cas, and also establishes evolutionary connections between
various types of rotating magnetized neutron stars in Be-binaries.Comment: 6 pages, accepted for publication in MNRA
Neglecting the porosity of hot-star winds can lead to underestimating mass-loss rates
Context: The mass-loss rate is a key parameter of massive stars. Adequate
stellar atmosphere models are required for spectral analyses and mass-loss
determinations. Present models can only account for the inhomogeneity of
stellar winds in the approximation of small-scale structures that are optically
thin. This treatment of ``microclumping'' has led to reducing empirical
mass-loss rates by factors of two and more. Aims: Stellar wind clumps can be
optically thick in spectral lines. We investigate how this ``macroclumping''
impacts on empirical mass-loss rates. Methods: The Potsdam Wolf-Rayet (PoWR)
model atmosphere code is generalized in the ``formal integral'' to account for
clumps that are not necessarily optically thin. Results: Optically thick clumps
reduce the effective opacity. This has a pronounced effect on the emergent
spectrum. Our modeling for the O-type supergiant zeta Puppis reveals that the
optically thin H-alpha line is not affected by wind porosity, but that the PV
resonance doublet becomes significantly weaker when macroclumping is taken into
account. The reported discrepancies between resonance-line and
recombination-line diagnostics can be resolved entirely with the macroclumping
modeling without downward revision of the mass-loss rate. Conclusions:
Mass-loss rates inferred from optically thin emission, such as the H-alpha line
in O stars, are not influenced by macroclumping. The strength of optically
thick lines, however, is reduced because of the porosity effects. Therefore,
neglecting the porosity in stellar wind modeling can lead to underestimating
empirical mass-loss rates.Comment: A&A (in press), see full abstract in the tex
On the Absence of Non-thermal X-ray emission around Runaway O stars
Theoretical models predict that the compressed interstellar medium around
runaway O stars can produce high-energy non-thermal diffuse emission, in
particular, non-thermal X-ray and -ray emission. So far, detection of
non-thermal X-ray emission was claimed for only one runaway star AE Aur. We
present a search for non-thermal diffuse X-ray emission from bow shocks using
archived XMM-Newton observations for a clean sample of 6 well-determined
runaway O stars. We find that none of these objects present diffuse X-ray
emission associated to their bow shocks, similarly to previous X-ray studies
toward Oph and BD433654. We carefully investigated
multi-wavelength observations of AE Aur and could not confirm previous findings
of non-thermal X-rays. We conclude that so far there is no clear evidence of
non-thermal extended emission in bow shocks around runaway O stars.Comment: 6 pages, 2 tables, 3 figures; Accepted to ApJ Letter
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